1. Field of the Invention
This invention relates to a power transmission device.
2. Description of the Related Art
When various mechanical apparatuses (driven apparatuses), such as conveyers, joints of industrial-use robots, and elevators, are driven, a power transmission device is interposed between a driving source and the mechanical apparatuses. The power transmission device is used to diminish the rotational speed of power outputted from the driving source, to increase the torque, or to change the axial direction of rotation. An internally meshing planetary gear mechanism is known as a structure forming the power transmission device. The internally meshing planetary gear mechanism includes external gears and internal gears between which a difference in the number of teeth is slight.
As a concrete structure of the internally meshing planetary gear mechanism, proposals have been made of an oscillating internally-meshing structure in which one of an external gear and an internal gear oscillates with respect to the other one (see U.S. Pat. No. 6,440,030, for example) or a flexible meshing type internally-meshing structure in which a flexible external gear meshes with an internal gear while being flexed by an oval wave generator disposed inside in the radial direction.
Meanwhile, in the power transmission device applied to, for example, a conveyer or a hoisting machine, such as a winch or a sheet shutter, a function in which an output shaft does not rotate in an opposite direction, is required in spite of the fact that the external force such as gravity or human power is imposed from the side of a driven apparatus. This is termed a so-called backstop function.
As the power transmission device having the backstop function, many proposals have been made of a device that performs this function by use of a backstop bolt or a brake. However, a problem resides in the fact that a special mechanism to fulfill the backstop function is needed separately from the device body, and therefore the device will be enlarged or costs will be raised. Therefore, various proposals have been made of a power transmission device with a self-locking function in which a power-transmitting mechanism itself has the backstop function.
A power-transmitting mechanism that uses worm gears can be mentioned as a typical power-transmitting mechanism that has the self-locking function. However, a problem resides in the fact that the transmitting mechanism that uses worm gears has low transmission efficiency and great energy loss (i.e., great power consumption). Generally, a power-transmitting mechanism having the self-locking function has low transmission efficiency without being limited to the transmitting mechanism that uses worm gears.
Furthermore, in order to improve the self-locking function, the rotational resistance of members that constitute the power transmission device is usually set to be high. This adjustment improves the self-locking function, however, concurrently brings about a result of increasing the rotational resistance in the positive direction. In other words, operating efficiency in a normal operation has a tendency to be reduced proportionately with the enhancement of the self-locking function. A motor with a greater capacity is needed to obtain a strong driving force in the positive direction while having a high self-locking function. Therefore, power consumption becomes higher correspondingly therewith.
That is, the relationship between the enhancement of the self-locking function and the improvement of operating efficiency (i.e., reduction in power consumption) is like two sides of the same coin. In actuality, the fact is that a design is prepared in accordance with the characteristics of a concrete device while finding a compromise therebetween.
Besides this, design requirements in conflict with each other exist between high torque and high transmission capacity both of which are needed when activated or when accelerated (i.e., when heavily loaded) and low noise and less vibration both of which are needed when steadily operated (i.e., when lightly loaded).
These mutually conflicting requirements can be concurrently satisfied by automatically changing a transmission path or a transmission gear ratio of a power-transmitting mechanism in accordance with a situation by use of an automatic clutch or an automatic gear-shifting mechanism. However, if so, the device will become more complex, and costs will be inevitably raised.
The present invention has been made in consideration of these circumstances. It is therefore an object of the present invention to provide a power transmission device especially capable of automatically satisfying the mutually conflicting requirements mentioned above with a simple structure and at low costs.